1. Field of the Invention
The present invention relates to an ion source which produces a plasma and extracts an ion beam from the produced plasma, a method of operating the ion source and an ion source system having the ion source. More particularly, the present invention relates to means for keeping temperature of a plasma production chamber for producing the plasma at low temperatures at the time of plasma production and means for operating the ion source selectively in a low temperature operation mode and a high temperature operation mode for the plasma production chamber.
2. Description of the Related Art
FIG. 4 shows an example of a related art ion source. An ion source 2 comprises a plasma production section 4 which ionizes ion species such as a gas or vapor introduced into the plasma production section 4 to produce a plasma 14. The plasma production section 4 is supported by a plurality (usually 4) of bar-like supporting members (support poles in this instance) on the basis of an ion source flange 36.
The ion source flange 36 is used for mounting the ion source 2 on a vacuum chamber which is called anion source chamber. A vacuum atmosphere is produced on an inner side of the ion source flange 36 (the plasma production section 4 side when the ion source 2 is mounted onto the vacuum chamber). The ion source flange 36 includes packings 38 for vacuum sealing, and has a water-cooling structure for cooling and protecting the packings 38.
The plasma production section 4 is called Bernas-type in this instance, and includes a plasma production chamber 6 for producing the plasma 4 therein, a filament 10 for emitting electrons and a reflector 12 for reflecting electrons. The plasma production chamber 6 has an ion-extracting aperture 8. The filament 10 and the reflector 12 are oppositely disposed within the plasma production chamber 6. The plasma production section 4 may be of another type, for example, a Freeman type which includes a bar-like filament. An ion beam 16 can be extracted from the plasma production section 4 (exactly, the plasma production chamber 6) under an electric field.
A material gas 20 as ion species (also called an ionizable material: The same shall apply hereinafter.) may be introduced into the plasma production chamber 6 via a gas introducing pipe 18, in this instance. The ion source 2 includes a vapor generating chamber (oven) 22 which heats a solid material 26 by a heater 28 to vaporize it into a vapor 24. The vapor 24 generated from the solid material 26 can also be introduced as ion species into the plasma production chamber 6 via a nozzle 23. The vapor generating oven 22 is supported by the ion source flange 36 through a support part 30 and an oven flange 32.
The plasma production chamber 6 is heated to high temperatures, for example, several hundreds ° C. to 1000° C., with production of the plasma 14. Such a heating of the chamber is caused by heat generated from the filament 10 and heat by an arc discharge generated between the filament 10 and the plasma production chamber 6.
The ion source flange 36 is cooled to have low temperature of about room temperature for protecting the packings 38, etc., as described above.
To cope with this, a related art technique uses a plurality of bar-like supporting members (support poles) 34 in order that the plasma production chamber 6 is mechanically supported by the ion source flange 36, and thermal conduction from the plasma production chamber 6 to the ion source flange 36 is kept low, while the plasma production chamber 6 is kept at high temperatures.
In a case where the ion species constituting the material gas 20 and the vapor 24 is a material of a high melting point, such as indium, indium fluoride or antimony, it is preferable to keep the plasma production chamber 6 at high temperature. Accordingly, no problem arises in the related art structure stated above. In the case of ion species, such as phosphorous and arsenic, for which the plasma production chamber is preferably kept at medium temperatures, the related art structure creates no problem.
In a case where the ion species constituting the material gas 20 and the vapor 24 is a material of which the melting point and sublimation point are low and which will undergo thermal dissociation of molecule at high temperatures, such as decaborane (B10H14), the following problem arises. When the plasma production chamber 6 is heated to have a high temperature at the time of plasma production, the number of decaborane ions in the produced plasma becomes small while the number of dissociation molecule ion, such as pentaboran ions or octaborane ions in the produced plasma becomes larger. Thus, the decaborane ion beam with a predetermined amount cannot be extracted.
Such a problem occurs not only when where the material gas 20 is introduced from the gas introducing pipe 18 but also when the vapor generating oven 22 is operated to generate the vapor 24. The reason for this is that the vapor generating oven 22 and the plasma production chamber 6 are connected by the nozzle 23. Hence, temperature of the vapor generating oven 22 increases undesirably due to the thermal conduction from the plasma production chamber 6 even if the current fed to the heater 28 of the vapor generating oven 22 is reduced or stopped. The temperature of vapor generating oven 22 also increases undesirably due to heat radiated from the plasma production chamber 6.
When the decaborane is used for the ion species, a large current beam of low energy is equivalently produced by utilizing the feature of the cluster ion beam, and ion beam irradiation (for example, ion injection) with less charge-up of the substrate is advantageously obtained. However, when the decaborane is used for the ion species, in particular temperature of the plasma production chamber 6 at the time of plasma production must be kept low. It must be kept at a temperature value below a range from room temperature to about 100° C., for example. However, it is almost impossible for the related art ion source 2 to achieve such low temperatures of the plasma production chamber 6.